The present invention relates to electrolytic cells comprising polymeric composition electrode and electrolyte members and to a method of economically making such cells. In particular, the invention relates to rechargeable lithium battery cells comprising an electrode-intermediate polymeric separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making laminar cells in which the electrodes take the form of layers comprising compositions of lithium and other compounds capable of intercalating lithium ions, and wherein an inter-electrode membrane or layer comprises a plasticized polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on porous separator structures or membranes which physically entrained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric were saturated with solutions of a lithium compound, such as LiClO.sub.4, LiPF.sub.6, or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes provided the necessary Li.sup.+ ion mobility for conductivities in the range of about 10.sup.-3 S/cm.
Subsequent developments, such as described in U.S. Pat. No. 5,296,318, provided electrolytic battery cells which have both positive and negative electrodes comprising compounds capable of intercalating ions and include strong, non-porous, flexible polymeric electrolytic cell Separator membrane materials which contain lithium salt electrolyte solutions and remain functional over temperatures ranging well below room temperature. These electrolyte membranes are employed either as separator elements with mechanically assembled battery cell components or in composite battery cells constructed of successively coated layers of electrode and electrolyte compositions. In each of these implementations, the polymeric electrolyte/separator elements often contain the lithium electrolyte salts at the time of cell assembly and, due to the hygroscopic nature of those salts, necessitate extraordinary environmental conditions during cell assembly.
More recent developments have provided a manner of utilizing these improved polymeric electrolyte membrane and electrode compositions which substantially eliminates the need for special environmental controls during cell manufacture. Typically, the polymeric electrode and electrolyte/separator layers are thermally bonded to form a laminated cell structure which ensures optimum interlayer functionality and enables the postponement of sensitive electrolyte incorporation until the final stages of battery construction or even later in its application as an activating fluid.
The laminate layer structure of these cells also provides a ready means for incorporating electrical current collector elements, usually as additional conductive layers or foils which can add further strength to the cell assembly. In order to provide optimum access of activating electrolyte solution to the electrode and separator layers, it is usually preferred that at least one of these collector layers, when comprising a normally impermeable material such as metal foil, be of an open grid or mesh structure, perforated, or otherwise of similar reticulate form to allow fluid permeation.
Since the capacity of laminar battery cells varies with electrode area, it is desirable to maximize that parameter in structuring battery products. However, obvious practical limitations dictate that overall linear battery dimensions be minimized. Thus, present battery fabrication operations strive to compact the laminar cell into a structure of minimum volume while maintaining the desired inter-electrode area. A common method of achieving such a balance has been to roll thin, flexible cell elements into a spiral coil, such as in U.S. Pat. No. 4,929,519, which, in effect, compacts the longitudinal dimension of the cell. A disadvantage suffered in this practice is the necessity of interleaving an insulating layer within the coil to prevent the direct contact of the opposite pole electrodes with resulting disfunction of the cell. Not only does such a practice necessitate the handling of a further battery element, it also results in the added weight and volume of a nonproductive battery component.
Another common expedient entails the use of multiple folds of individual electrode and separator elements, such as in U.S. Pat. No. 4,761,352, which unfortunately often results in similar unproductive redundancy of a significant portion of electrode and collector materials.
The present invention obviates such disadvantages by providing an improved form of laminar battery structure which can be fashioned of the preferred polymeric electrode and electrolyte elements while utilizing economical spiral fabrication techniques which are unhampered by the necessity for inter-electrode insulation elements. Further improvement is provided by a significant reduction in the expanse of conductive collector elements and in the amount of non-productive battery weight represented by such elements.